Depth of field is a measurement of the range of depth along a view axis corresponding to the in-focus portion of a three dimensional scene being imaged to an image plane by a lens system. Several parameters of a lens system influence the depth of field of that lens system. In general, optical systems with high magnification, such as microscopes, have small depths of field. Also, optical systems which use large aperture lens systems to capture more light generally have small depths of field.
In some situations it is desirable to have the benefits of a larger depth of field without giving up those optical qualities which generally result in small depths of field. For example, some analyses of microscopic specimens would be aided by the availability of a high magnification microscope with a relatively large depth of field. Such a microscope could be used to more clearly image the full structure of a microscopic object which is three dimensional in nature. Ordinary microscopes generally allow the clear viewing of a thin section of such a three dimensional specimen, due to the small depth of field of those microscopes. Portions of the specimen which are on either side of the in-focus section will be out of focus, and will appear blurry. The ability to clearly see the full three dimensional structure of a specimen would aid in the understanding of the structure of that specimen. This would be especially useful when used in conjunction with biojective microscopes which allow a user to view a specimen stereoscopically.
Another situation in which a small depth of field can pose problems is the low light photography of a scene with large depth variations. An example of this is a landscape scene including foreground objects photographed at night. In order to get sufficient light onto the film at the image plane of the camera, a large aperture lens must generally be used. A large aperture lens, however, will result in a relatively small depth of field. Because of the small depth of field, only a portion of the scene being photographed will be in focus.
A conventional method of imaging the depth information of a three dimensional microscopic scene is confocal microscopy. In confocal microscopy a single photodetector is situated behind a pinhole in an opaque screen. An objective lens focuses light from an illuminated point onto the pinhole, and the screen masks out any non-focused light. The illuminated point is generally illuminated by an intense, focused light source, such as a laser. The illuminating light source and the pinhole must be scanned over a microscopic specimen, either one point at a time or in a series of lines, in order to build up information for the whole region of interest. Depth information can be extracted from the data recorded by the photodetector. The information obtained from a confocal microscope can be used to image the three dimensional structure of microscopic specimens, but such a system is too complex and expensive for typical microscopy. Also, confocal microscopy is limited to situations in which microscopic specimens are being imaged, and is not practical for imaging macroscopic scenes.
What is needed is a system capable of producing an image of a three dimensional scene with enhanced focus over a large depth of field, without sacrificing optical qualities which ordinarily require a small depth of field.